By ALEXANDRA STEVENSON and MATTHEW GOLDSTEIN

January 23, 2014

The man whose shadow has been hanging over the insider trading trial of Mathew Martoma, the former SAC Capital Advisors trader, took center stage on Thursday when the prosecution’s top witness said F.B.I. agents told him the government’s true target was Steven A. Cohen, SAC’s billionaire founder.

The witness, Sidney Gilman, an 81-year-old doctor, testified that when agents for the Federal Bureau of Investigation first questioned him about providing inside information to Mr. Martoma, they said they really were after Mr. Cohen.

“I am only a grain of sand, as is Mr. Martoma,” Dr. Gilman said that the F.B.I. agents told him at his first meeting with them in September 2011.

“They said they are really after a man named Steven A. Cohen,” Dr. Gilman, a former University of Michigan medical professor, testified.

Dr. Gilman’s statement came in response to questioning by one of Mr. Martoma’s lawyers, Richard Strassberg, about why he initially lied to the F.B.I. about supplying inside information to Mr. Martoma.

Mr. Martoma is on trial in what federal prosecutors have called the most lucrative insider trading case on record, one that they say helped SAC avoid losses and generate profits totaling $276 million. The case against Mr. Martoma is part of an investigation over a decade into the workings of SAC and Mr. Cohen, regarded by many in the industry as one of the most successful stock traders of his generation.

A month earlier, a 12-member jury found another former SAC employee, Michael S. Steinberg, guilty of insider trading. The firm itself pleaded guilty to security fraud charges and agreed to pay a $1.2 billion penalty late last year. Mr. Cohen has not been charged with any wrongdoing but he faces a civil administrative action of failure to supervise that was filed by the Securities and Exchange Commission.

The introduction of Mr. Cohen into the trial almost came by accident on Thursday. After initially answering a question from Mr. Strassberg by saying the only person the F.B.I. asked him about was Mr. Martoma, Dr. Gilman said he wanted to say some more.

“Can I get back to the question because I didn’t quite answer it fully,” Dr. Gilman asked.

Mr. Strassberg initially objected, but the trial judge, Paul G. Gardephe of Federal District Court, said he would allow Dr. Gilman to finish his answer. It was then that Dr. Gilman made his statement about Mr. Cohen.

Jonathan Gasthalter, a spokesman for Mr. Cohen, declined to comment.

In two and a half days of testimony for the prosecution, Dr. Gilman told the jury that he repeatedly gave Mr. Martoma confidential information over two years, including the results of a clinical trial for an experimental Alzheimer’s drug in July 2008.

Mr. Martoma is charged with using that inside information to avoid substantial losses in shares of Elan and Wyeth, which were jointly developing the drug.

Dr. Gilman was a member of the safety monitoring committee for Elan during the second phase of its clinical trial. In light of his expertise, he was also a consultant in high demand with hedge fund clients of the Gerson Lehrman Group, an expert network firm.

The doctor is testifying against Mr. Martoma after reaching a nonprosecution agreement with the government in September 2012, two months before Mr. Martoma was arrested outside his home in Boca Raton, Fla.

In cross-examination, Dr. Gilman admitted that he lied in early sessions with F.B.I. agents and prosecutors but said he was now telling the truth.

Mr. Strassberg suggested that Dr. Gilman began to cooperate with the government after becoming aware of how much jail time he could face. But the doctor said the prospect of potentially dying in prison and being financially ruined did not terrify him.

At that point, Mr. Martoma’s wife, Rosemary, seated in the front row of the gallery, muttered to herself but the comment could not be heard.

The doctor showed signs of a patchy memory when he was unable to recall dozens of meetings with other hedge funds he consulted with. He testified that even though he met with analysts and traders working for hedge funds like Citadel, Caxton Associates, Magnetar Capital and Maverick Capital, as well as money managers at JPMorgan Chase and Putnam Investments, the only meetings he recalled with clarity were those with Mr. Martoma.

“I don’t remember these names; just a few stand out in my memory,” he said in response to questions about other consultations he had done.

In a line of questioning that became tense at times, Mr. Strassberg asked Dr. Gilman about his other consulting work in an attempt to cast doubt on the credibility of his detailed testimony about his dealings with Mr. Martoma.

Mr. Martoma was introduced to Dr. Gilman in January 2006 by Gerson Lehrman. He testified that he earned more money as a consultant for Gerson Lehrman than he did as a professor.

Under questioning from Mr. Strassberg, Dr. Gilman acknowledged that some of his memory of early meetings with Mr. Martoma was refreshed when he was reviewing his calendar appointments in meetings with prosecutors.

“I never said I recalled that first consultation. I only said it happened because I saw the documents,” he said of his initial phone call with Mr. Martoma.

Dr. Gilman also displayed a spotty memory of an October 2006 meeting arranged by Gerson Lehrman for him to meet a number of hedge fund clients, including SAC. It was during this visit that Dr. Gilman, testifying under questioning by the prosecution, said that he first met Mr. Martoma.

To illustrate how little Dr. Gilman remembered of his other consultations, for which he was paid on average $1,000 an hour by Gerson Lehrman, Mr. Strassberg questioned the doctor about a meeting he had with an analyst from a small hedge fund in June 2008.

For roughly half an hour, Mr. Strassberg repeatedly asked Dr. Gilman about specific details of their conversation concerning the clinical drug trial. To each question, the doctor responded that he could not recall, though he did not deny something may have been said.

“I may have, but I don’t recall doing so, sir,” Dr. Gilman responded at one point.